Animate objects following a grease pencil stroke?
I don't think it is possible to use a Grease Pencil stroke directly as animation path.

What you could try instead is convert the desired grease pencil stroke to a Bezier Curve object and use it instead.

If you need the original grease pencil try converting a copy of it instead, so you can keep both intact.

This answer describes how you can do it in more detail.